Once you can install arbitrary programs on a device (or a single program, the firmware) you should treat these devices with a general mistrust. The main proble

Once you can install arbitrary programs on a device (or a single program, the firmware) you should treat these devices with a general mistrust. The main problem here that you can't use your usb devices at public places safely, since they can patch your usb device's firmware at will. The problem here that you use your trusted usb device on a not-trusted computer which can do anything it wants with your device's firmware. There should be an approval mechanism directly, physically on the device to prevent this. For example on usb thumb drives there could be a switch on the device which turns it to "firmware upgrade mode".